You need to upgrade all your system, the only thing you may keep is the HDD.
1- New PSU- Depending on the quality of your actual PSU.
3-NEw MOBO, CPU AND RAM
- What´s your PSU name? Amps in 12 V rail??
I won´t buy a 9800 gt, try to get something made in 40 nm, like:
- ATI 5670
- Geforce GT 240
What´s your budget?